The present invention relates to the kind of puzzles generally known as jigsaw puzzles, consisting of a plurality of pieces which can be assembled to form a picture, first applied on a support, the whole being cut afterwards into pieces. This picture represents any kind of scene, generally the reproduction of a painting, a drawing, or a photograph.
More particularly, the present invention relates essentially to jigsaw puzzles cut by a number of lines extending into two directions in the puzzle plane, approximately orthogonal to each other, in which the pieces are arranged in such a way that each piece is placed in a position at the intersection of a row and a column, so that each of its four edges can be directly in contact with only one edge from a single other piece. Each piece of the puzzle is therefore adjacent to at most four other pieces. Generally, the cutting lines are slightly irregular, a reminiscence of the time when puzzles were cut manually, but the pieces can be also cut in a perfectly regular way.
Still more particularly, the present invention relates to jigsaw puzzles in which the pieces are not only placed side-by-side, but also are joined together by means of a plurality of male and female interlocking elements, which are part of the edges of the different pieces, and ensure that each part of the puzzle remains attached to the whole in the puzzle plane, no other means being necessary to this end, so that the pieces can be only assembled and disassembled in a direction outside the puzzle plane. In most cases, two adjacent pieces are joined together by a male element, which is part of the edge of the first piece, and a female element, which is part of the edge of the second piece, these elements, generally of rounded shape, ensuring that the two pieces remain interlocked. In a very few cases, two adjacent pieces are joined together by several male and several female interlocking elements. In some cases, the cutting line between two adjacent pieces is just irregular, the other pieces ensuring that these two pieces remain attached together. Traditionally, the arrangement of these male and female elements is free, the different pieces having a variable number of male elements and a variable number of female elements, but, in many cases, the male elements alternate with the female elements, for practical reasons.
Some pieces of the puzzle, named outer edge pieces, form the border of the puzzle, some edges of these pieces defining the perimeter of the puzzle. In most cases, this perimeter is of rectangular shape, which only makes possible a true representation of a rectangular picture, as well as an easy framing of the puzzle once assembled, but this perimeter can be also of any other shape. The other pieces, named interior pieces, form the interior part of the puzzle. Generally, such a rectangular puzzle comprises (m.times.n) pieces, including (2.times.(m+n)-4) outer edge pieces, each having one straight edge, except the four corner pieces, each having two adjacent straight edges, and ((m-2).times.(n-2)) interior pieces, without any straight edge.
Traditionally, when puzzles where cut manually, the cutting style was not always well defined, but, on the contrary, could be free and irregular. For example, the cut would follow the line separating two sections of the picture with different colors, the only help to assemble the puzzle being the shape of the pieces. Today, the mass production of puzzles is mechanized and standardized. The same die is generally used to cut different puzzles, representing all kinds of pictures. The difficulty of assembling of such a puzzle depends essentially on the number of pieces, the average size of the pieces, and the complexity of the picture itself.
Another approach to increase the difficulty of assembling of a puzzle is disclosed in U.S. Pat. No. 5,022,655, in which the cutting lines of the puzzle include one or several rectangles, or more generally one or several polygons, so as to add new straight edge pieces on both sides of these rectangles, or polygons, as well as new corner pieces with two straight edges. As the outer edge pieces are not any more distinguishable, by their shape, from these interior pieces, a result of this approach is to make the assembling of a puzzle slightly more difficult.
However, in practice and in any of its different embodiments, this approach requires a certain number of traditional interior pieces without any straight edge, which does not make it possible for this patent to meet its objective entirely and really increase the difficulty of assembling of a puzzle. Moreover, a major disadvantage of this approach is to produce a puzzle made of several separated parts, that are not attached to each other, which results in the weakening of the structure of the puzzle. In order to solve this problem, this patent suggests to join the different separated parts of the puzzle by a number of special "joiner" pieces, made of two ordinary pieces joined together, crossing the boundaries between these different parts. The use of similar double pieces has been suggested before in French Pat. No. 2,395,051, as a way to join two puzzles placed side by side to form a larger puzzle. This technique, even if it provides a minimum way to ensure that the different parts of the puzzle remain attached to each other, is nevertheless artificial, as it requires the use of double pieces, completely different by their nature from the other puzzle pieces. For all these reasons, the approach described in this patent is not entirely satisfactory.